Marten Gunderson is a veteran of countless conflicts worldwide, Gunderson runs the Agency. This is a thinly disguised mercenary recruitment service providing specialized forces for anything from basic security to invasions.
He was a member of The Cabal and he was a personal bodyguard to Pieter Van Eckhardt, he seemed to be under the control of Eckhardt as he blinked excessively when he had physical contact with Eckhardt. Gunderson seemed to despise Dr.Grant Muller and Kristina Boaz as he always criticized them and he always regarded them as useless as their only responsibility was to create the Proto-Nephilim and other creatures (although these were actually large responsibilities).
Later, when a now mutated Boaz is released to kill Lara Croft and Kurtis Trent, he, Eckhardt and Grant Muller are standing on a raised platform. He waits till Eckhardt finishes his speech and when all three of them are about to leave, Eckhardt shouts at Muller saying "Muller, you useless piece of dross, you failed me too!" Then Gunderson pushes Muller over the edge, who is then killed by Boaz. He then leaves and that is the last time Gunderson is seen.
It is unknown what happened to Gunderson as he was not killed by Lara in the Tomb Raider: The Angel of Darkness. Perhaps he will turn up in another Tomb Raider game but that would be unlikely.